1. Field of the Invention
The present invention relates generally to a printed circuit board, a printed circuit board module, and an electronic device adapting same, and more particularly, to a printed circuit board, a printed circuit board module, and an electronic device adapting same in which the power supply layer is divided into a plurality of lands.
2. Description of the Related Art
Electronic devices typically contain one or more printed circuit board modules, in which a variety of electronic elements are mounted atop a printed circuit board. However, in such a configuration, electromagnetic waves radiating from the printed circuit board can cause the electronic device to malfunction. Accordingly, in order to improve the reliability of such devices it is necessary to restrict the radiation of these electromagnetic waves from the printed circuit board.
In order to further an understanding of the problem the present invention attempts to solve a description will now be given of noise currents.
When the electronic elements operate, noise such as switching noise is generated. Hereinafter, the current caused by such noise is referred to as noise current.
It is known that the intensity of these electromagnetic waves is related to the size of the area of the return loop through which the noise current flows, so that the larger the area enclosed by the loop the stronger the electromagnetic wave radiation. It should be noted that the term xe2x80x9creturn loopxe2x80x9d refers to the closed path along which the noise current flows from the noise source back to the noise source.
Accordingly, in order to suppress the electromagnetic waves radiating from the printed circuit board, it is desirable that the area enclosed by the loop-like return path through which the noise current flows be as small as possible. It should be noted that the return path through which the noise current flows is the path of least load resistance. Therefore, it is desirable that such a path enclose the smallest possible area.
Additionally, electronic devices have come to be loaded with an increasing number of functions to make data processing faster. In order to accommodate these added demands it has become necessary to mount electronic elements of different operating voltages on the printed circuit board. These operating voltages may for example be 5 V, 3 V, and 2.8 V.
An additional requirement particularly with respect to portable electronic devices is that battery power be conserved and the life of the battery extended. In order to accomplish this aim, the printed circuit board is mounted with a plurality of circuits having different functions. Power is then turned ON not to the entire printed circuit board but only to each of those circuits requiring power, and the power is turned OFF with respect to those circuits whose operations are not required, thus saving battery power.
It will be appreciated by those skilled in the art that, in order to be able to supply power selectively, that is, to selected portions of a printed circuit board mounted with electronic elements having different operating voltages, the printed circuit board must have either a plurality of power supply layers or a single power supply layer divided into a plurality of lands that are electrically insulated from each other.
Providing a plurality of power supply layers complicates the structure of the printed circuit board, and reduces the production yield of the printed circuit board. Accordingly, the conventional solution is to divide the single power supply layer on the printed circuit board into a plurality of mutually electrically insulated lands.
FIG. 1 shows perspective and cross-sectional views of a conventional printed circuit board 10 and a portion of a conventional printed circuit board module 20. The drawing at the right of FIG. 1 is the cross-sectional view of the printed circuit board 10. The drawing at the left of FIG. 1 is a schematic rendering of the printed circuit board 10 drawn to emphasize a power supply layer 11, a dielectric layer 12 and a ground layer 13. The drawing at the right represents a cross-sectional view along a line Ixe2x80x94I of the drawing at the left.
As shown in the diagram, the portion of the printed circuit board 10 shown in FIG. 1 has the power supply layer 11 divided into a plurality of mutually electrically insulated lands A, B and C. The dielectric layer 12 and the ground layer 13 are provided beneath the power supply layer 11. Another dielectric layer 14 and a signal layer 15 are provided above the power supply layer 11. The dielectric layer 12 disposed between the power supply layer 11 and the ground layer 13 forms the land A and the ground layer 13 into a capacitor Ca, the land B and the ground layer 13 into a capacitor Cb and the land C and the ground layer 13 into a capacitor Cc. A voltage Va is applied across the ground layer and the land A, a voltage Vb is applied across the ground layer and the land B, and a voltage Vc is applied across the ground layer and the land C.
As shown in the diagram, electronic elements are disposed atop the lands having the appropriate operating voltages. An electronic element 30, for example, has an operating voltage Vc and is thus mounted on the land C. Similarly, an electronic element 31 has an operating voltage Va and is thus mounted on the land A.
Normally, an electronic element, like the electronic element 30 shown in FIG. 1, has all its terminals positioned atop the same land. However, some electronic elements, such as the electronic element 31 shown in FIG. 1, electrically straddle two lands, in this case land A and land B. As a result, a ground terminal Pg31 of electronic element 31 to be connected to the ground layer 13 is positioned not so as to oppose the land A but so as to oppose the land B.
As shown in FIG. 1, NS30 is the source of noise generated by the operation of the electronic elements as described above, in this case electronic element 30. Similarly, NS31 is the source of noise generated by the operation of the electronic elements as described above, in this case electronic element 31. A noise current i30 from the noise source NS30 flows through a loop-like return circuit indicated in the drawing by reference numeral 40 via the ground layer 13 through the capacitor Cc to the 30 land C and back to the noise source NS30. As can be seen from the drawing, the area enclosed by this return path 40 is extremely small and hence the electromagnetic waves generated from the noise source NS30 are weak.
By contrast, consider a noise current i31 from the other noise source NS31 and the path along which it returns to the noise source NS31. As noted above, the land B is electrically insulated from the land A. The noise current i31, flows along a return path indicated by reference numeral 41, that is, from the ground layer 13 to the capacitor Ca to the land A and back to the noise source NS31. The area enclosed by this loop is large and hence the electromagnetic waves generated from the noise source are strong.
Accordingly, the above-described printed circuit board 10 and printed circuit board module 20 are not capable of adequately suppressing electromagnetic wave radiation.
It should be noted that although FIG. 1 shows the capacitors Ca, Cb and Cc as a single line connecting two opposed layers, in actuality the entire areas of the opposed layers or plates form the capacitors Ca, Cb and Cc. The same holds true of FIGS. 2 and 4 which will be described later.
Additionally, although the loop-like return path shown on the left of FIG. 1 would appear to be three-dimensional, in actuality a direction in a thickness of the printed circuit board has been exaggerated compared to a horizontal direction of the printed circuit board for purposes of illustrative clarity only. It is understood by those of skill in the art that in actuality the direction in the thickness of the path is so small that it can be ignored and that the area enclosed by the loop-like return path is essentially a flat surface.
FIG. 2 shows perspective and cross-sectional views of another conventional printed circuit board 10A and printed circuit board module 20A. The drawing at the left of FIG. 2 is of the printed circuit board 10A, drawn to emphasize the power supply layer 11, the dielectric layer 12 and the ground layer 13. The drawing at the right of FIG. 2 represents a cross-sectional view of the printed circuit board 10A at left along a line Ixe2x80x94I. It should be noted that elements shown in FIG. 2 that are corresponding or identical to elements shown in FIG. 1 are given identical or corresponding reference numbers, with detailed descriptions thereof omitted.
The printed circuit board 10A shown in FIG. 2 is essentially the same as the printed circuit board 10 shown in FIG. 1, with the addition of bypass capacitors 50-1 through 50-4 each having a capacitance of from 100 to 1,000 pF. Bypass capacitors 50-1, 50-2 straddle the lands A and B. Bypass capacitor 50-3 straddles the lands B and C. Bypass capacitor 50-4 straddles the lands A and C.
The noise current i30 from the noise source NS30 related to electronic element 30 flows along the same return path 40 described above and returns to the noise source NS30.
The noise current i31 from the noise source NS31 related to the electronic element 31 flows along a return path indicated by reference numeral 51, that is, from the ground layer 13 to the capacitor Cb to the land B to the bypass capacitor 50-1 to the land A and then to the noise source NS31. This return path 51 encloses an area smaller than the area enclosed by the return path 41 described above, and accordingly, the printed circuit board 10A and the printed circuit board module 20A shown in FIG. 2 show improved electromagnetic wave suppression characteristics as compared to the printed circuit board 10 and printed circuit board module 20 shown in FIG. 1.
However, the printed circuit board and printed circuit board module having the structure described above have the following drawbacks.
First, as electronic devices have become more compact the size of the printed circuit board 10A has decreased, making it physically more difficult to mount the bypass capacitors used to suppress the electromagnetic wave radiation. Additionally, use of the bypass capacitors takes up space and restricts the placement of other electrical elements, which in turn affects the layout of the wiring of underlying layers because the wiring must be positioned beneath the electronic elements.
Second, and as one consequence of the limitations on design freedom described above, the bypass capacitors cannot always be positioned so as to create loops having minimal enclosed areas. As a result, adequate electromagnetic wave radiation suppression cannot always be obtained.
Accordingly, it is an object of the present invention to provide a printed circuit board, a printed circuit board module, and an electronic device adapting same, in which the problems described above are solved.
The above-described object of the present invention is achieved by a printed circuit board comprising:
a ground layer;
a power supply layer divided into a plurality of lands;
a dielectric layer disposed so as to cover the plurality of lands of the power supply layer; and
a conductor layer disposed so as to cover the dielectric layer,
the plurality of divided lands being coupled to each other by electrostatic capacitors formed by each of the lands of the power supply layer and the conductor layer sandwiching the dielectric layer therebetween.
According to this aspect of the invention, the loop formed by the noise current return path is reduced as is the area enclosed by the loop, thus suppressing electromagnetic wave radiation form the printed circuit board.
Additionally, the above-described object of the present invention is also achieved by the printed circuit board as described above, further comprising:
a high-resistance conductor layer which covers the surface of the conductor layer,
the high-resistance conductor layer being electrically coupled to the ground layer.
According to this aspect of the invention, an electrical charge accumulating on the high-resistance conductor layer is released to the ground layer, so that no charge is accumulated in the high-resistance conductor layer.
Additionally, the above-described object of the present invention is also achieved by the printed circuit board as described above, further comprising:
a high-resistance conductor layer which covers the surface of the conductor layer,
the high-resistance conductor layer being electrically coupled to the lands of the power supply layer.
According to this aspect of the invention, an electrical charge accumulating on the high-resistance conductor body is released to the lands of the power supply, so that no charge is accumulated in the high-resistance conductor layer.
Additionally, the above-described object of the present invention is also achieved by the printed circuit board as described above, further comprising:
a capacitor and a resistance which are coupled in series between the conductor layer and the ground layer.
According to this aspect of the invention, harmonics of the electromagnetic waves radiated from the printed circuit board are suppressed even if these coincide with the resonance frequencies of the printed circuit board.
Additionally, the above-described object of the present invention is also achieved by a printed circuit board module comprising:
a printed circuit board,
the printed circuit board comprising:
a ground layer;
a power supply layer divided into a plurality of lands;
a dielectric layer disposed so as to cover the plurality of lands of the power supply layer; and
a conductor layer disposed so as to cover the dielectric layer,
the plurality of divided lands being coupled to each other by electrostatic capacitors formed by each of the lands of the power supply layer and the conductor layer sandwiching the dielectric layer therebetween.
According to this aspect of the invention, electromagnetic wave radiation from the printed circuit board is suppressed.
Additionally, the above-described object of the present invention is also achieved by an electronic device comprising:
a printed circuit board module,
the printed circuit board module comprising a printed circuit board,
the printed circuit board comprising:
a ground layer;
a power supply layer divided into a plurality of lands;
a dielectric layer disposed so as to cover the plurality of lands of the power supply layer; and
a conductor layer disposed so as to cover the dielectric layer,
the plurality of divided lands being coupled to each other by electrostatic capacitors formed by each of the lands of the power supply layer and the conductor layer sandwiching the dielectric layer therebetween.
According to this aspect of the invention, electromagnetic wave radiation from the printed circuit board is suppressed. Additionally, electrostatic discharge can be avoided, thereby improving the reliability of the electronic device.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.